The invention relates to obtaining parts made of composite material comprising a carbon reinforcing fabric densified by a matrix formed by chemical vapor infiltration (CVI).
A particular field of application for the invention is that of obtaining parts made of thermostructural composite material comprising carbon fiber reinforcement densified by a matrix of carbon or ceramic. Such parts are used in the fields of aviation and space, and also for friction elements, in particular for brake disks.
The reinforcing fabric is typically obtained using carbon precursor fibers such as preoxidized polyacrylonitrile (PAN) fibers, pitch fibers, phenol fibers, or rayon fibers, all of which withstand the textile operations needed to shape such fabrics better than do carbon fibers.
The carbon precursor fiber fabric is transformed into a carbon fiber fabric or preform by applying heat treatment. On an industrial scale, the heat treatment is performed in an oven at substantially atmospheric pressure while being swept with an inert gas such as nitrogen. Temperature is raised progressively up to about 900° C. The transformation of the precursor into carbon is almost total, with the resulting carbon content generally exceeding 95%, and possibly reaching 99% or more. The loss of mass is considerable, being about 50%, and is accompanied by a large volume of gaseous effluent being produced.
For at least some applications, it is necessary not only to transform the precursor into carbon, but also to perform subsequent heat treatment at high temperature, in particular in order to eliminate metals or metallic impurities coming from the precursor and/or in order to confer special properties on the carbon fibers. This applies in particular to eliminating the sodium contained in preoxidized PAN, which sodium can have a harmful effect on the ability of the resulting composite material parts to withstand oxidation.
That is why, following a first carbonizing step and prior to densifying the fabric, preoxidized PAN carbon precursor fiber fabrics are sometimes subjected to heat treatment at high temperature and under reduced pressure in order to eliminate sodium by sublimation. This second step is performed under low pressure while sweeping with an inert gas such as nitrogen, and at a temperature that is generally higher than 1000° C., typically lying approximately in the range 1400° C. to 1650° C. in order to eliminate sodium, and possibly reaching 2000° C. or 2200° C. or even 2500° C. in order to eliminate other metallic impurities and/or to transform the properties of the fibers.
Steps of carbonizing, performing heat treatment at high temperature, and subsequent densification by chemical vapor infiltration are conventionally performed in respective special purpose installations. In industrial use, each of these steps lasts for several days. This explains why, in particular, the process of obtaining sodium-free composite material parts containing fiber reinforcement made using preoxidized PAN precursor is lengthy and expensive.
The same problems arise with carbon fibers coming from precursors other than preoxidized PAN and likewise containing sodium or other metals for elimination, such as magnesium or calcium, and also whenever it is necessary to eliminate metals or metallic impurities such as iron, nickel, or chromium, for example, which require heat treatment at high temperature, typically up to not less than 2000° C. or 2200° C., or even 2500° C. in order to be eliminated by sublimation.